Modern klystron amplifiers typically have used pencil-thin electron beams, and pill-box shaped resonant cavities to bunch the and extract electromagnetic radiation from the beams. However, because in such a configuration the electron beam is confined to such a small volume, the space charge effects and beam loading limit the total charge and total energy such a beam can carry, and hence limit the klystron's power output. To overcome this, klystrons have been developed at the Naval Research Laboratory which have a hollow beam in a hollow drift tube. This configuration reduces space charge depression, and, because the beam in this configuration is generally cylindrical, rather than pencil-like, such a configuration can carry higher total current for the same current density. The hollow drift tube configuration is limited in that the frequency of operation must be below the cutoff frequency of the drift tube. This limits the radius of the drift tube. Because the radius is limited, the amount of current is limited by space charge effects. It appears that such configurations are limited to power of about 30 GW. It is believed that one reason for this apparent limit is that prior high power klystron tubes have a large electric potential between the electron beam and the walls.